Recently, strong demand for the reduction of environmental load and environmental protection on a global scale urges automobiles to discharge cleaner exhaust gases to reduce the amounts of air-polluting materials, to save energy, and to improve fuel efficiency to reduce the amount of CO2 discharged, which is one of causes for global warming Developed and adopted for the cleaning of exhaust gases from automobiles and improving fuel efficiency are various technologies, such as engines with higher performance and fuel efficiency, post treatments for removing air-polluting materials discharged from engines, the weight reduction of automobile bodies, the reduction of aerodynamic resistance of automobile bodies, low-loss power transmission from engines to driven systems, etc.
Among them, technologies for providing engines with higher performance and fuel efficiency include direct fuel injection, high-pressure fuel injection, an increased compression ratio, higher boosting pressure of turbochargers, reduced displacement, the weight and size reduction of engines by turbo-charging, etc., and these technologies are applied not only to luxury cars but also to popular cars. As a result, combustion tends to occur in engines at higher temperature and pressure, so that combustion chambers of engines discharge a higher-temperature exhaust gas to exhaust members. For instance, even popular cars have as high exhaust gas temperatures as 1000° C. or higher like luxury sport cars, resulting in exhaust members having surface temperatures of higher than 950° C. Exhaust members are put in a severer oxidation environment than ever, being exposed to oxidizing gases and oxygen in the air in such a high temperature region. Exhaust members are also repeatedly subjected to cycles of heating and cooling by the start and stop of engines. Thus, exhaust members are required to have higher heat resistance and durability such as oxidation resistance, thermal fatigue life, etc. than desired conventionally.
Because exhaust members such as exhaust manifolds, turbine housings, etc. for gasoline engines and diesel engines of automobiles have complicated shapes, they have conventionally been produced by casting with high degree of design freedom. In addition, because they are used in severe conditions at high temperatures, heat-resistant cast iron such as high-Si, spheroidal graphite cast iron and Ni-Resist cast iron (austenitic, cast Ni—Cr iron), heat-resistant, ferritic cast steel, heat-resistant, austenitic cast steel, etc. having excellent heat resistance and oxidation resistance are used.
Although conventional heat-resistant cast irons such as high-Si, spheroidal graphite cast iron and Ni-Resist cast iron have relatively high strength at exhaust gas temperatures of 900° C. or lower and at exhaust member temperatures of about 850° C. or lower, they have lower strength and heat resistance such as oxidation resistance and thermal fatigue life, when exposed to an exhaust gas environment exceeding 900° C. Further because Ni-Resist cast iron contains about 35% by mass of Ni, a rare metal, it is expensive. The heat-resistant, ferritic cast steel usually has poor high-temperature strength at 900° C. or higher.
The heat-resistant, austenitic cast steel is more durable to high temperatures than the heat-resistant cast iron and the heat-resistant, ferritic cast steel. JP 7-228948 A discloses heat-resistant, austenitic cast steel suitable for exhaust members, etc. of automobile engines, which comprises by mass 0.2-1.0% of C, 0.05-0.6% of (C—Nb/8), 2% or less of Si, 2% or less of Mn, 15-30% of Cr, 8-20% of Ni, 1-6% of W, 0.5-6% of Nb, 0.01-0.3% of N, 0.01-0.5% of S, the balance being Fe and inevitable impurities. JP 7-228948 A describes that heat-resistant cast steel obtained by adding proper amounts of Nb, W, N and S to 20Cr-10Ni-based, heat-resistant, austenitic cast steel has improved high-temperature strength at 900° C. or higher, as well as excellent castability and machinability, suitable for exhaust members.
However, because the 20Cr-10Ni, heat-resistant, austenitic cast steel described in JP 7-228948 A was proposed to be used at exhaust member temperatures of about 900-950° C., it has poor heat resistance and durability, and insufficient oxidation resistance and thermal fatigue life, at temperatures of about 1000° C. Particularly its thermal fatigue life is unsatisfactory, leaving room for improvement. Accordingly, it cannot be used for exhaust members (for instance, turbine housings for turbochargers operable at high boosting pressure), whose surface temperatures reach about 1000° C.
As an exhaust member made of heat-resistant, austenitic cast steel having improved durability in high-temperature use conditions, JP 2000-291430 A discloses an exhaust member made of heat-resistant, high-Cr, high-Ni, austenitic cast steel having a composition comprising by mass 0.2-1.0% of C, 2% or less of Si, 2% or less of Mn, 0.04% or less of P, 0.05-0.25% of S, 20-30% of Cr, and 16-30% of Ni, the balance being Fe and inevitable impurities, and further comprising 1-4% of W, and/or more than 1% and 4% or less of Nb, a mass ratio of Cr/Ni being 1.0-1.5. The heat-resistant, high-Cr, high-Ni, austenitic cast steel described in JP 2000-291430 A is cast steel obtained by controlling the composition range and structure based on 25Cr-20Ni, heat-resistant, austenitic cast steel with increased amounts of Cr and Ni as main alloy elements, rather than on the 20Cr-10Ni, heat-resistant, austenitic cast steel to have drastically improved high-temperature strength and oxidation resistance, such that it can be suitably used for exhaust members exposed to exhaust gases exceeding 1000° C. (particularly about 1050° C., further about 1100° C.).
However, the 25Cr-20Ni, heat-resistant, austenitic cast steel described in JP 2000-291430 A contains large amounts of Cr and Ni, expensive rare metals, to have high-temperature properties and heat resistance. Because these rare metals are produced in small amounts only in limited countries and regions, they are not only expensive but also easily influenced by global economic conditions, resulting in unstable supply, high price by speculation, etc. Because the 25Cr-20Ni, heat-resistant, austenitic cast steel described in JP 2000-291430 A contains about 25% by mass of Cr and about 20% by mass of Ni, it suffers high production cost, economically disadvantageous for use in exhaust members of engines for popular cars, and fails to secure stabile supply.
To clean exhaust gases from automobiles and improve fuel efficiency, exhaust members should be improved in various points in addition to the above heat resistance and durability. For instance, in a post-treatment of cleaning exhaust gases (a treatment for removing harmful materials, etc. from the exhaust gas by a catalyst and a filter in an exhaust-gas-cleaning apparatus), it is necessary to improve cleaning performance by quickly activating the catalyst by temperature elevation when the engine is started, and by supplying an exhaust gas uniformly to the entire catalyst and filter. For quick activation of the catalyst, an exhaust gas passing through the exhaust member should be subjected to little temperature decrease, namely, the energy of the exhaust gas should be kept as much as possible. Accordingly, exhaust paths should have small thermal capacity (heat mass), requiring that the exhaust member is as thin as possible. To improve the cleaning performance of a catalyst, etc., to prevent engine power from lowering, and to improve the efficiency of turbochargers, etc., the exhaust gas should flow smoothly with little pressure loss. To this end, it is effective to provide the exhaust gas with reduced flow resistance, improved distribution, less turbulence and interference, etc. For instance, the design of an exhaust member should take into consideration the shortening of exhaust paths, the prevention of rapid direction changes, etc.
Automobile bodies are required to have a reduced weight and low aerodynamic resistance for higher fuel efficiency, and improved safety. For instance, a body has a low bonnet immediately above an engine room for improved aerodynamics, an impact-absorbing (crushable) zone in the engine room for safety at car crash, etc. These measures have decreased the freedom of layout design in the engine room, requiring that the exhaust member has reduced weight and volume. Thus, to make automobiles lighter and safer, weight and size reduction, smooth discharge, etc. are necessary to exhaust members.
To meet the above requirements for exhaust members, proposals have been made to provide, for instance, (a) a thin, light-weight exhaust manifold having exhaust paths with small thermal capacity, which comprises exhaust path members that are branch pipes formed by thin metal plates or pipes, and cast members including flanges fixed to a cylinder head, a turbine housing, etc. and a converging case, both members being welded to each other, (b) a long exhaust manifold comprising pluralities of cast members welded via corrugated pipe members for preventing cracking due to thermal expansion, (c) a light-weight, small-size exhaust member comprising a cast exhaust manifold and a cast turbine housing, which are welded instead of usual fastening with bolts to make thick flanges for fastening bolts and space for inserting a bolt-fastening tool unnecessary, thereby reducing thermal capacity, etc.
As described above, to achieve high heat resistance and durability, as well as thinning, weight and size reduction, smooth discharge, etc. necessary for exhaust members, it is effective to weld plate or pipe metal members with cast members, or cast members themselves. With respect to exhaust members that tend to have complicated shapes, cast members with high freedom of shape are contained and welded to improve their freedom of design and ease of production, thereby omitting parts such as fastening bolts, gaskets, etc.
Necessary for welded exhaust members is sufficient weldability to avoid weld cracking. The weldability is an important property not only for welding members but also for repairing defective cast members by welding, affecting their production yield and productivity. Thus, materials for exhaust members are required to have weldability in addition to heat resistance and durability. With respect to the heat-resistant, austenitic cast steel, JP 7-228948 A and JP 2000-291430 A do not provide enough investigation as to the improvement of weldability in addition to heat resistance and durability, taking economic feasibility into consideration.